Wave signaling system



June 5, 1934 w. A. MacDONALD WAVE SIGNALING SYSTEM Original Filed July21, 1930 3 Sheets-Sheet l INVENTOR WA Mania/mid,

,MJM ATTORNEYS June 5, 1934.

W. A. M DONALD WAVE- SIGNALING SYSTEM Original Filed July 21, 1930 '5Sheets-Sheet 2 l J E INVENTOR WA, Macflonapla? Y L mb/m ATTORNEYS I.zmmtum m Q x mkm m Patented June 5, 1934 UNITED STA TES PATENT o'rncs1,961,154 wave SIGNALING sits-11 2M William A. MacDonald, Little Neck,N. Y., assignmto Hazeltine Corporation, a corporation of DelawareOriginal application July 21, 1930, Serial No. 469,487. Divided and thisapplication December 12, 1931, Serial No. 580,619. In Canada Juneapplication Serial No. 469,487, filed .luly'izlst,v 1930, and issued May23, 1933 as United States I Patent No. 1,910,399.

A primary object of the invention is to provide 'a radio receiver of thetype specified embodying the features of av gang. operated multipleattenuator volume control for varying the signal intensity over a widerange of values in a substantially distortionless manner; an overall.sensitivity response characteristic varying with frequency in a mannerwhich is under control of the designer; an overall selectivity which isalso under the control of the designer but which preferably ismaintained substantially constant throughout the tuning range; and animproved audio-irequency characteristic so shaped as tocompensate atleast in part for the side-band. attenuation of the higheraudio-frequencies caused by the selectivity characteristics of theradio-frequency tuning system. 1

Detailed objects are to provide improved coupling circuits forinterconnecting the antenna 4 and first thermionic tube or forconnecting successive tubes in cascade relation. The individual couplingcircuits are adapted to slope the voltage amplification curve 'for agiven stage-in-a desired manner whichZis under control of the designer.

Inthe radio-frequency stages the individual coupling circuits areadapted further to control automatically the variation in resonance bandwidth with frequency over the tuning range inv accordance with aselected design. In the aggregate, the coupling circuits produce a finalsensitivity-response curve of preselected slope,

and an audio-frequency characteristic of substantially constantefficiency over the essential range of frequencies iior speech andmusic.

A refinement consists in coupling the last radio-frequency amplifiertube to the detector by means of a coupling circuit having a highervoltage amplification ratio than the coupling systems associated withthe preceding stages in order to prevent the tube preceding the detectorfrom overloading.

Additional features oi the invention will become apparent from thesubsequent detailed description:

when read in conjunction with the appended drawings. r

- tion.

25 Claims. (01. 178-44l In connection with the coupling circuits de-'scribed herein, the term eflfective or resultant coupling will be used.This will be understood as the total coupling electrostatic as well aselectromagnetic existing between the portions of a circuit underconsideration.

. The invention will be best understood by immediate reference to thedrawings, wherein Figure 1 shows diagrammatically a complete radiobroadcast receiving system inaccordance with the present invention, andFigure 2 shows a similar view of a modified circuit.

Figures-3, 4 and; show various forms of coupling circuits applicable tothe present invenbased.

Referring to the drawings, the receivers com prise an antenna circuit 1,four thermionic tubes V1 to V4, inclusive, operatingas stages ofradiofrequency amplification and a detectdr tube V5, all of thescreen-grid -heater-element type, in-

terconnected in cascade relation by, means on coupling systems orcircuits B, C, D and E, respectively with the first tube suitablycoupled to the antenna circuit through coupling system or circuit A. InFigure 1, a coupling network F connects the detector output to the inputof the .first audio-frequency amplifier tube Vs,the output of which isconnected through the trans? former '1; to a pair of power tubes V7 andVa operating push-pull. The output circuits of the power tubes areconnected in opposition through the transformer T9 to aloud speaker LS.In Figure 2,'the first audio-frequency amplifier tube v. is omitted, thedetector output being coupled directlytothe power tubes through thecoupling circuit E terminating in the primary winding of transformer Ta.

The radio-frequency coupling circuits A to E, inclusive, .are tunableover a range in frequency by means of the variable condensers C1 to C6,inclusive, which are gan'g operated by a uni-control device, or controlmember, U1 adapted to simultaneously adjust all circuits to the sameresonance frequency.

Each screen grid'tube is provided with anode 2, cathode 3, grid 4,screen'grid 5, and filament 6.

The filaments 6 operate in the well-known fashion to heat the cathodes 8to the desired operating temperature and to this end may e supplied withcurrent from any suitable source. The filament supply circuits areomitted from the drawings for the sake of clarity, inasmuch as they arewell understood in the-art andconstitute no part in the presentinvention.

The anodes 2 and the screen grids 5 of the radio-frequency tubes aremultiplied toconductors 7 and 10. respectively, having suitable'sourcesconnected thereto for applying the necessary operating potentials to thetube electrodes. The cathodes 3 are grounded .through the commonresistance 11 which due to the fiow of space current therethrough raisesthe cathodes to a positive potential above ground and is equivalent tonegatively biasing the grids by the same amount since the latter aregrounded.

With the form of bias provided by the potential drop in resistance 11,the negative bias on 'the effect of random variations in-the mutualconductance of commercial tubes employed in the receiver. Due to thevery high radio-frequency amplification employed in receivers of thetype disclosed herein, it, has been found that the random variationsfrom average of the mutual conductance of the tubes is suflicient toincrease the receiver sensitivity by a factor of two or three ascompared to the sensitivity obtained with the average tubes, withresultant increased tendency of the receiver toward regeneration andoscillation.

By employing the automatic bias in the form of resistance 11, if themutual conductance of the tube is increased, this change in--tubecharacteristic will have little or no effect on the overall receiversensitivity since the tendency-for an in-. crease in. plate currentresultant uponthe increased mutual conductance will immediately producean opposing effect in the form of an increased negative bias on the gridtending to reduce the mutual conductance of the tube. The net effectwill be that a very slight readjustment in plate current will compensatefor a rather wide variation from average in the mutual conductance ofthe tube with consequent stabilization in the receiver operation.

Tubes Vs-Va, inclusive, are supplied with proper operating potentialsover circuits similar to those supplying tubes Vl-V4 and these circuitsaccordingly will not be described in any detail since the circuits areobvious from the drawings.

For the tubes carrying high-frequency current,

. the low potential point of the anode, grid and screen grid circuitsare coupled.- to the corresponding cathodes through the high-frequencylay-passing condensers 12, which serve to prevent undesirable couplingeffects between the input sistors 8 interposed in the battery leadsextending to conductor '7. These resistors in conjunction with theassociated by-passing condensers l2'serve as sections of resistancefilters preventing the passage of high-frequency currents. In a similarmanner the screen grid circuits are with the antenna, may have aninductance suflito respond most strongly to the lower frequencies itycoupling aids or opposes the magnetic couisolated by means of theresistors 9 interposed in the leads extending to conductor 10cooperating with the associated by-passing condensers 12 to form filtersections. The values of the -re- 'The electrical coupling. system Acomprises a pair of coupled tunable circuits interposed in cascaderelation between the antenna and tube V1 for selecting and translatingthe desired signal with a high degree of'discrimination. The antennaandground constitute a. pair of input terminals and the grid and cathode oftube V1 constitute a pair of output terminals, for the coupling system.The nature and magnitude of the couplings between the pair of tunable,or adjustably resonant, circuits .are such as to cause" the overallsensitivity .and selectivity to vary in a desired manner over thetunable frequency -range. Thus, to obtain substantially uniformsensitivity, i. e., constant voltage amplification as the frequency oftuning is varied, the primary coil P1 of transformer T1, included in thecircuit ciently large in comparison with the antenna, capacity that thenatural periodicity of the antenna circuit is lower than the lowestfrequency to be received. This causes the antenna circuit within thetuning range and to discriminate against the higher frequencies, therebyoffsetting wholly or in part the factors which are operative, in thewell-known manner, to,cause the voltage" amplification from the primaryto the tunable secondary circuit of the transformer T1 to increase withthe frequency of tuning.

By properly selecting the coupling between the pling. Capacity Blpreferably comprises the inherent capacity existing between the primaryand secondary turns of the transformer windings but a physical capacitymay, of course, be used for this purpose. If the capacitive couplingaids the magnetic coupling, the voltage amplification will tend toincrease with the frequency, whereas for the two couplings opposed thedesign may .be such as to cause the voltage amplification to decreasewith frequency.

The resonant secondary circuit coupled to the antenna circuit comprisesthe secondary winding S1 of transformer T1 included in a closed seriescircuit with the primary winding P2 of a second transformer T2 and thevariable condenser C1 for adjusting the frequency of tuning. Theresonant secondary circuit S 1C1P2 is coupled through a link circuit toa second tunable circuit comprising the secondarywinding S2 oftransformer T2 and the variable condenser C2 connected thereacross. Thelink circuit comprises the magnetic coupling existing betweenthe'windings P2 and S2 together with the electrostatic coupling providedby condenser K1 extending between the high potential points of the twotunelectrostatic coupling increases with the fre-' quency of tuning, theresultant eifect produced in the resonant circuit S2--C2 may by properproportioning and poling of the elements be caused to vary both theselectivity and sensitivity in a desired manner with frequency. If, forexample,

it is desired to obtain a more nearly uniform resonance band width overthe tuning band than ls obtainable with the usual type of resonantcircuit, the pair of couplingsof the combined electrostatic andelectromagneticcoupling means should be arranged to oppose each other.Under this condition, the dual coupling means produce in the resonantsecondary circuit S2-C2 dualistic reactions which vary oppositely withfrequency. Further, the coupling at the low-frequency end of the bandshould be adjusted to optimum or slightly more than optimum coupling,the adjustment being such as to provide a resonance curve having adesired band width. As the frequency of tuning increases, the opposedrelations of the electrostatic and the electromagnetic couplings willcause a condition of less than optimum coupling to be obtained for thehigher frequencies, thereby tending to increase the sharpness of tuning.0n the other hand, the

natural increase with frequency of the load or effective .resistance inthe resonance circuit S2-C2 due, for example, to skin effect,eddy-currents and the like, works in the opposite direc-' tion, tendingto broaden the resonance curve. These two effects may by suitable designbe caused to balance one another and thereby to produce effects sovariable with frequency as to maintain a substantially constant degreeof selectivity, of resonance band width, over the en tire frequencyscale. The selectivity, or band width, is approximately the same whenthe system is tuned to a frequency adjacent the upper limit of thetunable range as when tuned to a frequency adjacent the lower limit ofthe tunable range.

Inasmuch as with the arrangement described immediately above, theeffective coupling be-- tween the first and second. tunable circuitsautomatically decreases with increase in tuning frequency, theefficiency of energy transfer between circuits likewisedecreases in thesame manner and thus tends to produce a corresponding variation in theoverall voltage amplification ratio of the coupling circuit. In order tooffset this efiect, the coupling between primary and secondary circuitsof transformer T1 is adjusted to accentuate the. voltageampliflcationtoward the high-frequency" end of the tuning range. In thisway there is obtained an approximate balance of voltage gains in the twotuned circuits, as regards variation with tuning frequency, whichresults in an approximatelyflat gain characteristic as well as the abovenoted approach tp uniform selectivity'over the tuning range.

To attain the desired results, the capacity K1 need not necessarily beconnected between the high potential points of adjacent tuned circuits,

but may be tapped to an intermediate point of either coil S1 or coil .82or to intermediate points of both coils.

The set of curves in Figure 6 are illustrative of tuning in thewell-known manner.

- 3 J the results that may by proper design be obtained with a couplingnetwork such as circuit A of Figure 1. Figure 6 gives the results oflaboratory measurements made on a circuit of the type shown in Figure 7and although the actual measurements are for the coupling networkinterposed between a pair of thermionic valves, it will be apparent thatthe same results are obtainable when the connection is between anantenna circuit and a thermionic valve as is the case for circuit A.

Referring 'now to Figure 6, curve M1 shows the variation with thefrequency of tuning of the voltage amplificationas measured from theinput section of tube Vx, Figure '7, to the first tuning condenser Cx.The design wassuch as to produce the rising amplification characteristicwith frequency shown in order, as explained above, to offset the effectof decrease in coupling with frequency between the first and secondtuned circuits S 1-Cx and S2--C in order to provide substantiallyuniform overall amplification or sensitivity.

Curve M2 shows the gain characteristic or overall sensitivity for thecomplete network as measured from the input section of tube Vx to theinput section of tube V for the condition that the electrostaticcoupling Ky opposes the magnetic coupling P3S2, the relativeproportioning of the and less-than optimum coupling atthe higherfrequencies in order to insure substantially uniform selectivity, or atleast, minimize changes in slectivity. It is apparent from curve M2 thatthe. overall sensitivity is practically constant through the entiretuning range.

Curve N1 is a measure of the total resonance band width athalf-amplitude as measured across the first tuning condenser Cx; whilecurve N2 gives corresponding measurements across the second tuningcondenser Cy. Curve N2 which is a measure of the overall selectivity issubstantially fiat,

varying less than 20% over the tuning range.

It will be apparent from a study of the results in Figure 6 thatacoupling network of the type shown in Figure 'l is admirably adaptedfor use in high-frequency systems since it permits the designer tocontrol simultaneously the manner in which both sensitivity andselectivity vary with frequency.

Returning now to Figure 1, the nature of the.

requirements for voltage amplification and selectivity may be such thatthe coupling circuit A, as described, does not fully meet all conditionsso that the alternative circuits may be found pref erable. For example,a modification that may be utilized is to arrange the primary P1 oftransformer T1 instead of being of high inductance to be of lowinductance, thus causing the voltage amplification to increase with thefrequency of posing the capacitive coupling K1 to the magnetic couplingPz-St' and properly proportioning the magnitudes of these couplingeffects, the gain or voltage ratio of the tuning system may be,maintained very flat, even though the effective Then by op-- couplingbetween the two tuned circuits decreases toward the higher frequencies.-

In Figure 2, three couplings are provided for circuit, or system, A"that become effective in greater or less degree, depending upon thefrequency of tuning. .'I'he magnetic coupling Pz-Sz is substantiallyconstantover the tuning range. The eflect produced by. capaci y K: andresistance R: in shunt therewith included in resonant circuit S2C2 is toprovide an effective change in coupling between the'two tuned circuitsover the tuning range, the coupling provided by these elemerits beinggreatest in the low frequency range.

In addition, the energy dissipating impedance means K-Rz tends toincrease the power factor of the tuned circuit S?-C2, and hence of thecoupling system, at the lower frequencies, thus providing widerresonance bands at such frequencies than are normally obtainable. As thefrequency of tuning increases the elfective magnitude of resistanceinserted in the resonance circuit.S2-Cz by the impedance K2 Rzautomatically decreases, due to the by-passing effect of capacity K2.This variation in the losses or, dissipative effect, introduced by theimpedance K-Rz, it will be observed, is opposed to the normal mode ofvariation of the resistance in the resonance circuit Sz-C2 introduced byskin efiect eddy current losses and the like, which factors ure 2, thecapacity coupling K1 may or maynot be employed and may be arranged toeither aid or oppose the direct capacity coupling due to the condenserK2, or may be made to aid or oppose the magnetic coupling Pz-Sz, orythecapacitive coup ings may be used exclusively, in which case the coils P2and S2 may be positioned to'provide no magnetic coupling therebetween.

The antenna circuit contains a variable resistance R1 connected in shuntwith the primary winding of the transformer T1. This resistanceconstitutes a volume control for adjusting the signal intensityimpressed upon the receiver.- I

-Byir'1terposing the double-tuned circuit A between the antenna and thefirst amplifier tube, the desired signal may be chosen with such a highdegree ofselectivity that the system may be rendered substantiallyopaque to undesired extraneous signals while at the same time selectingtin? desired signal with'minimum attenuation, thereby preventingintermodulation eifects.

Directing attention now to the first of the interstage couplingnetworks, the output circuit of the first radio-frequency amplifier tubeV1 is in,

'stantf ally fiat voltage amplification characteristic over the tunablerange. Tothis end the coils are so made that the distributed capacitybetween turns aifords natural periodicities within the tunable range.The transformeris further arranged to provide a small voltage step-upbetween primaryand secondary circuits, and also have not too high amutual inductance between windings,

the magnetic coupling being such as to introduce the two eaks in theresonance curves adjacent the upper and lower limits of the tuning bandrespectively. By winding the coils P and S of high resistance wire or byconnecting a resistance such as R: across the primary winding, the peaksof the resonance curve may be flattened out to such an extent that thetransformer provides substantially uniform response over the tunablefrequency range.

An alternative design is to provide a magnetic coupling between primaryand secondary windings,,sufliciently close to introduce but a singleresonance peak within the tuning range, and to properly damp this peakby means of the winding resistance or the shunt element R3 to aiforduniform voltage amplification.

The preferred design of this transformer is to construct the windings inseveral pies to minimize the distributed capacity between turns. These'pies may comprise self-supporting coils held on an insulating mandrelor may be wound in slots cut in a separate form. The form may beenclosed in a metal cup to minimize the extent of 1 external field. 4 Itis preferable that an iron cup be employed for this purpose since itincreases the losses in the transformer circuit which, as pointed 'outabove, are desirable in this instance.

A second volume control in the form of a variable resistance R4 isconnected in shunt with the secondary winding of the transformer Ta, Thevolume control R4 is simultaneously adjust- ,able with the control R1 inthe antenna circuit by means of a uni-control device U2 mechanicallycoupling the variable elements of these two resistances. In this way itis easily possible to secure an attenuation of 45 d. b. per control or'a total attenuation of d. b. This form of control is especiallydesirable as it provides ample signal attenuation without altering thebias- .ing potentials applied to the tube electrodes and thus permitsthe tubes to be operated at the most favorable portions of theircharacteristics at all times.

Y The use of a coupling circuit B for connecting the first and secondtubes, having, an untunable secondary circuit, provides severaladvantages,

former T3 permits the insertion of the second volume control elementRrat this point. .The volume control element R4, of necessity,introduces a certain fixed capacity to ground equivalent to about six,or more, micro-micro-farads into the secondary circuit of thetransformer, so that if a tuning condenser were connected in circuit atthis point the minimum capacity of this circuit would exceed that of theother resonant circuitsby the capacity of the volume control element.This would require that all of the other selecting circuits be padded upto the same minimum capacity, and would also necessitate the use oflarger and more expensive variable condensers than is required with thecircuit arrangement shown, since the tuning range covered by thevariable condensers is determined by the ratio of the maximum to theminimum capacity. It the minimum capacity is increased, the maximumcapacity must be increased in the ratio of the square of the upper andlower frequency limits in order to cover-a given tuning range. Thiswould mean, of course, that the able frequency range.

variable capacities would have to be constructed to have a much largermaximum capacity than required with the arrangement shown.

The coupling circuit 0 connecting the output of the tube V: to the inputof tube V3 may be designed to provide substantially constant voltageamplification and selectivity over the tun- The coupling transformer T4comprises a high inductance primary winding P1 and a low inductancewinding P2 of relatively few turns, both windings being magneticallycoupled to the secondary winding S. The primary winding P1 is shunted bya capacity K3 such that the circuit IQP1 has-a natural periodicitylower, but not greatly, lower,'than the lowest frequency within thetuning range. The circuit Kz-Pr is thus capacitively reactive over thetuning range with the result that as the frequency of the tuningincreases a smaller and smaller percentage of the total signalingcurrent flowing in the output circuit of tube V: will flow through thewinding P1, thereby in effect automatically decreasing the couplingbetween windings P1 and S as the frequency of tuning increases.

Winding P2 is so connected to the circuit Kr- P1 that the magneticeffects with the two primary windingsupon the secondary circuit areadditive. Since the coupling between the primary winding P2 and thesecondary winding S is substantially constant over the frequency rangewhile the coupling between the windings P1 and S automatically decreaseswith increased frequency, the ratio of the voltage in the secondarycircuit to the voltage in the primary circuit will rise with frequency,but at a rate which is under control of the designer and may be madesuch as to cause the amplification to remain substantially constant withfrequency changes, or to rise with frequency at a desired rate.

Capacity K3 is preferably small and may in certain cases comprise thedistributed capacity of the winding P1 and the anode-to-cathodecapacitential end of the form supporting the secondary winding S, or itmay be wound directly. over the low potential end of the secondarywinding but separated therefrom by the thin sheet of insulat ingmaterial such as varnishedcloth or celluloid.

"It is preferably to space the turns of winding P2 so that they have thesame pitch as those of the secondary winding S, for in this way thedistributed capacity between the two windings is reduced.

The secondary winding S is tuned to resonance by means ofvariablecondenser Ca and has included in the resonance circuit thecondenser K4 shunted by resistance R5. The impedance K4R5 offers ahigher resistance to the low frequencies than to the high frequencies.iBy proper selection of the magnitudes of K4 and R5- the amount ofresistance introduced into the tuned circuit may be made to varyautomatically over a wide range as the .frequency of tuning' changes, sothat it is possible to broaden the width of the resonance band or toincrease the power factor of the circuit at 'tlie' lowerfrequencies'without are coupled in cascade relation. Uniformselecmaterially affecting, or affecting only to a small degree, thepower factor at the higher frequency range. In this way it is possibleto secure almost any desired resonance band ratio at the two ends of thetuning range. The results of both measurements and computation indicatethat when employing a secondary circuit having a power factor of about1%, the maximum broadening effeet occurs when the resistance Rrin ohms.is

about of the same order as the capacity reactance K4 in microfarads.

The natural capacity B2 existing between the primary winding P1 and thesecondary winding 8 of transformer T4 may be arranged to aid or opposethe magnetic couplings between'windings and thus intensify or diminishthe magnetic coupling effects thereof as the frequency of tuningincreases.

Figure 8 is illustrative of the results obtainable with couplingcircuits such as network 0, the curves having the same significance asthose of Figure 6. 'The curves of Figure 8 represent the results oflaboratory measurements using the circuit' arrangement shown in Figure9.

Curve Ma shows the voltage amplification as measuredfrom the inputsection of tube VI: to the input section of tube Vy for the conditionthat the dissipative impedance Ky-Ry is short circuited; while curve M4gives the corresponding curve with the short-circuit removed, 1. e.,with the impedance KyRy included in the secondary circuit as shown inFigure 9. Curves N3 and N4 give the variations with frequency of theresonance band width at half amplitude as measured across condenser 0::for the conditions that Ky---Ry is short-circuited and is included inthe secondary circuit respectively. It will be seen from these curvesthat by proper design a coupling circuit is obtained which providessubstantially uniform sensitivity and selectivity through- 5 out thetuning range.

It will be seen from a comparison of Figures 7 and 9 and the associatedcurves of Figures 6 and 8 respectively that two separate and distincttypes of coupling circuits have been described for securing uniformselectivity throughout the tuning range. The circuit of Figure '7relates to a structure wherein a pair of tunable circuits tivity issecured by arranging the gain characteristic of the entire unit and theeffective coupling between tunable circuits in such a way thatover-optimum coupling is secured in the low-frequency range whileunder-optimum coupling is secured at the higher frequencies. 'Inpractice this results in a resonance curvefor the low-frequency rangewhich has been artificially broad-- ened in the region of resonancebyfrom 5 to 10 kilocycles on either side of the exact resonancefrequency, but which follows a 'normal 1 resonance curve severalchannels removed from the resonance frequency. At the higher fre-'quencies of tuning the complete resonance curve is, of course, of theconventional shape.

' This point is illustratedby the. curves of Figure 10 wherein curve Ishows the frequency-response' characteristic of the coupling circuitwhen tuned to a frequency near the upper frequency limit; while curve'Hgives the results for tuning near the low-frequency limit. Due to theeffect 5 of over-optimum coupling, curve Hhas' the well understooddouble hump in theregion of resonance which thus broadens the resonanceband at its base to substantially the, same width as is obtained withcurve I. The width .of-bands m H and I at resonance is, of course,designed to provide substantially uniform response with minimumattenuation over the essential band of audio-frequencies equivalent toone-half to one channel on either side of exact resonance. For widedepartures from exact resonance, such as six or seven channels, theresponse curves do not have to be and, in fact, are not the same, as isindicated by the upper portions of curves H and I. .The curves in thisregion are of the conventional character, with curve H narrower in thisregion than curve I due to the smaller losses occurring at the lowerfrequencies.

Now with respect to the second method of obtaining uniform selectivityover the tuning range, accomplished by insertion of the dissipativeimpedance KyRy, Figure 9, in the resonance circuit, the resultantartificial widening of the resonance band at the lower frequencies issecured by actually increasing the effective power factor of thetransformer which has the effect of broadening the resonance curvethroughout its entire extent at such frequencies; so that if theresonance curves for the upper and lower frequency limits of tuningcoincide at one channel from exact resonance, they will be of the sameorder of magnitude six or seven channels removed from resonance. Thus,referring to Figure 10, this resonance curve for tuning at the lowerfrequency will coincide with curve-I for the upper frequencies. Thisdistinction represents a slight difference in operating characteristicsbetween the circuits of Figures 7 and 9, although the net result will beof the same order for the'two circuits insofar as side band admission orelimination is concerned.

A double-tuned circuit D is employed in the arrangement of Fig. 1 toconnect the output ciras to produce additive effects in the secondarycircuit. The magnetic coupling between the'two primary windings and thesecondary is so proportioned as to give a desired slope in theamplification-frequency curve of the transformer. The secondary windingS1 is tuned to resonance by means of the variable capacity Ci'connectedbetween ground and the high potential end of the secondary, the lowpotential end thereof being connected through a low inductance primarywinding Prof transformer T6 to ground. The

secondary circuit of transformer T5 is coupled to tunable circuit S2C5through a link circuit comprising an electrostatic coupling by virtue'of capacity K5, extending between the high potential points of thesecondary windings S1 and S2, and an electromagnetic coupling by virtueof the mutual inductance existing between windingsPs and S2 If asubstantially uniform resonance band ratio is desired for the tuningrange the capacity coupling K5 can be arranged to oppose the magneticcoupling, and the two couplings combined can be arranged to optimum orjust over optimum coupling for the lowest frequency to be received. Themagnetic coupling should predominate throughout the range. Then with thecapacitive coupling opposing the magnetic the resulting coupling willdecrease asthe tuning frequency increases, thereby reducing the reactionof one resonant circuit upon the other to render the tuning sharper withincrease in frequency,

which effect opposes the natural tendency for the resonance curve tobroaden out at the upper frequency limit, the resulting effect beingsuch that the resonance band width is about the same throught thefrequency scale.

In order ,to insure that the coupling circuit D will-providesubstantially uniform voltage amp1i fication over the tuning range, itis necessary that transformer T5 be so designed that the voltageamplification for this transformer 'will increase with the frequency oftuning, as indicated in curve M1 of Figure 6, to an extent necessary tooffset the decrease in efliciency of energy transfer from the firsttothe second tuned circuit due to the automatic decrease in couplingtherebetween.

The coupling circuit E connecting the output of tube V4 to the input ofthe detector tube V5 is similar in construction and operation to thecoupling circuit C discussed above. It comprises the uniform gaintransformer T7 having a high inductance primary winding P1 tuned by theshunt capacity K11 to a frequency below the range, and a low inductanceprimary P2. Both primaries are magnetically coupled in an additive senseto the resonant secondary circuit containing in series relation thetuning condenser Cs, secondary winding S, andthe fixed-capacity Kcshunted .by resistance R6 for adiusting the resonance band width.

Coupling circuit E is preferably arranged so that it has a. highervoltage gain than the circuit just preceding it, in. order that powerfullocal signals,of low modulation percentage will not overload thetubepreceding this circuit before the fulloutput of the power tube'isobtained. This condition is very likely to occur where the de-.

tector works directly into the power tubes. For this samereason, itisdesirable that coupling circuit E be designed for uniform gain,particularly when the tube preceding the detector tube V5 isoperatingnear the overload point.

The detector tube .V5' is of the screen grid type operating-as aself-biasing tube due to connection of the cathode to ground throughresistance 13 which minimizes to a considerable degree the overload ofthe detector itself. The output of detector tube V5 is coupled by meansof circuit F to the input of a first audio-frequency amplifier tube Vs.Circuit F includes a resistance coupling network comprising resistancesR7 and Rs included in theanode and gridcircuits of tubes V5 and V6,respectively, the high potential terminals of the resistances beingconnected through a blocking condenser K7 while the low potentialterminals are connected through ground and the battery supply circuit.

Interposed between the output section of tube V5 andjresistance R7 is alow-pass filter section comprising the series connected radio-frequencychoke coil L1 and the shunt capacity Ks which particular filter shown ismerely intended to be indicative of the method employed for shaping theaudio-frequency characteristic. In some instances it may :be desirableto utilize a highpass or even a band-pass filter to attain a desiredresponse curve.

With receiving systems as normally constructed embodying the usual typesof coupling circuits for interconnecting the radio-frequency stages, theresonance band width increases with frequency of tuning due to theincrease with frequency in effective resistance of the tuned circuits,producing thereby corresponding variations with frequency in the widthof the side bands trans-' mitted. If the selectivity is such as totransmit at the lower frequencies of tuning a band width correspondingto the essential range of audiofrequencies, then at the higherfrequencies of tuning the band width'transmitted will be so broad as tointroduce extraneous signals from stations operating on other than thedesired wave length. If, on the other hand, the selecting circuits aredesigned to transmit. the essential range of audible frequencies whenthe tuning is adjusted for the upper frequency limit, marked limit dueto the increase in selectivity with reside band attenuationor'trimming-will occur as the. tuning is adjusted toward the lowfrequency frequency portion of the receiver may be designed tocompensate effectively for the side-band attenuation throughout thefrequency range. One method of shaping the audio-frequencycharacteristic in this manner is by means of the lowfrequency filter ,incoupling circuit F of Figure. 1.

Figure 11 is illustrative of what may be ac-,

.complished by way of shaping the overall frequency characteristic for areceiver of the type 'disclosed herein. Curve G1 which shows theresponse characteristic of the audio-frequency stages Veto Va,inclusive, is substantially flat over the essential range ofaudiblefrequencies extending from 11 to is. G2 shows the overallfldelity'characteristic of the receiver with the low-pass filter ofcoupling circuit F omitted and illustrates clearly the side bandattenuation introduced due to the selecting networks at thehigher audiofrequencies. response characteristic forthe audio frequency portion ofthe receiver including a high-passfilter in the coupling circuit Fsuitably designed to compensate in part for the trimming caused by theselecting networks. Curve G4 shows the final overall fidelitycharacteristic of the receiver with the high-pass filter included in thecoupling circuitd andthus shows clearly the manner in which -.theaudio-frequency characteristic has been improved. The curves of Figure11 are not'intended tobe rigorously correct, but rather are illustrativeof what may be accom-' plished by way of improving the overall receivercharacteristic. It .is to be stressed again that results such as shownin Figure 11 are obtain- 'able only where, as in the present invention,the

selecting networks, are such as to provide a .tances with larger sizedwire spacing the turns,

Curve Gs' shows the,

substantially uniform resonance band width throughout the tunable range.

The radio-frequency amplifier as a whole may be designed in several waysdepending upon the operating conditions. Assume, for example, it isdesired to employ tuned circuits having about the characteristics nowcommonly employed in broadcast receivers which have a power factor ofabout 1%, then by broadening the resonance band of the individualstages. at thelow-frequency range, while leaving .the resonance band atthe high-frequency range, substantially unaffected, the ratio over thewhole range will remain uniform. If the conventional number of three orfour tuned circuits were to be employed, this broadening of theresonance band niightimpair the overall selectivity, but when employingcoils having a 1% power factor, a larger number of tuned circuits willbe employed than has heretofore been the practice, so, in this way, thedesired overall selectivity can be secured.

Where a large number of tuned circuits are employed, theory andexperiment indicate that the peak of the resonance curve may be quitebroad while for conditions slightly off resonance the sides are verysteep. This steepness oi the sides of the resonance curve (just as atthe low frequency range of present commercial receiving the gaincharacteristic of the audio system atthe low frequency range such as byinterposition of the high pass filter circuit in the detector output orby increasing the high frequency response.

The present invention may be successfully practiced when employing theconventional number of three, four or five tuned circuits. by usingtuned resonant circuits of a lower power factor than 1%. This maybeaccomplished by employing larger diameter coils than now commonly usedand winding the secondary inducor using radio-frequency cable ratherthan solid wire. In this way it is possible to reduce the power factorof the tuned transformers to .5% or lower. This practice, whetheremployed with three or a greater number of tuned circuits, is likely toseriously impair the quality ofreproduc- -tion because of side bandcutting, but as previous-- ly described, the effects may easily becompensated for by proper shaping of the audio characteristic; 1

"In many cases it is desirable to reduce the high-frequency response ofthe audio system, particularly above 3000 or 4000 cycles. This may be.very'conveniently accomplished in the pres- 7 cut invention by properselection of the number and power factor of the tuned circuits so thatthe side-band attenuation of the radio-frequency portion of theamplifier becomes effective slope the gain characteristic so that it hasabout "sitivity of a receiveris high, it is preferable to. I

a 2:1 ratio; that is, it has about twice the sensi-.'

tivity at the low-frequencyv as at the high-frequency range. By slopingthe curve in this way, the stability and freedom from oscillation"remains about uniform. This condition may, of course, not be desirablefor all circumstances but may easily be modified as conditions warrant.Figure 2 shows a circuit designed in this manner.

In Figure 2 transformer T1 of coupling circuit A is provided with a lowinductance primary winding included in the antenna circuit, thus' of thevarious couplings and.the design of thev dissipative circuit K2'Rincluded in the resonant secondary circuit S2C2, is such as to providesubstantially uniform voltage amplification and constant selectivityover the tunable range.

'Coupling circuit B connecting tubes V1 and V2 in cascade relation inthis instance comprises,

'a uniformgain transformerT-i similar to. that of :coupling circuits Cand E of Figure 1. The volume control resistor R4 is in this instancebridged across the primary circuit of transformer T4 since, for reasonsexplained above, it is undesirable to have this element bridging thetunable secondary circuit. The connection of control R4 across theprimary circuit oftransformer T4 necessitates insulation offlall activeportions of the control above ground potential, .andin this respect doesnot provide so desirable an arrangement as the circuit of Figure 1.

It is, of course, not essential that transformer T4 be of the uniformgain type. By, employing such a transformer, however, it is possible tode-. sign the amplification characteristic to compensate for thecapacitive reactance introduced into the primary circuit. due toemployment of the .tends to equalize the impedance over the entirerange, thus producing uniform amplification.

The resistor R5,and shunt capacity K4, included in the resonant circuit,are selected of such values as to provide about the same or a slightlywider resonance band at the, low than at the high frequency tuningrange.

The broadly tuned aperiodic transformer T3 now comprising circuit Ccoupling the output of tube V2 with the input to tube V; may be given' avoltage amplification slope favoring the lowfrequency over thehigh-frequency response in about a 2:1 ratio in order to provide acorre- -C and E of Figure 1. of uniformgain coupling circuits areavailable sponding overall frequency-response characteris tie for theradio-frequency amplifier.

The double-tuned circuit D of Figure 2 employs a uniform gain type oftransformer T10 in the output circuit of tube V3, the gainfrequencycharacteristic of which maybe given a desired that preferably thenatural capacity K12 betweenturns in conjunction with the anode-cathodecapacity o'f tube Va is suflicient .to render. the pri- The primarywinding P1 is wound to a- 1,oe1,1s4 V mary circuit resonant at afrequency slightly be low the tuning range. If necessary, of course, aseparate condenser may be used to supplement 3 the natural capacity K1:in order to provide the required natural periodicity of the primarycircuit. The primary P1 is coupled magnetically to the secondary windingby means of the mutual inductance between windings and also capacitivelyby means of condenser K12 connecting the high potential terminals of thetwo windings.

The primary winding due to the capacity in shunt therewith iscapacitively reactive over the tuning range, and hence-has a fallingimpedance characteristic for increases in frequency, which efiectopposes the gain in voltage amplification with frequency, which in theabsence of other factors would exist. This opposing effect is furtherintensified due to the automatic decrease in effective coupling betweenthe primary and sec ondary circuits with increase in frequency resultingfrom the shunting effect of capacity K13.

The capacitive coupling K12 may be arranged, de-

pending upon the respective'polarities of the high potential-terminalsof the primary and second-' ary windings, to either aid oroppose themagnetic coupling in the secondary circuit, the particular arrangementutilized being determined by the desired slope of the gain-frequencycharacteristic to be attained. In the present instance it isdesirable tohave a substantially uniform frequency-gain characteristic for thetransformer T 0 and accordingly the capacitive coupling K12 is arrangedto aid the magnetic coupling, since otherwise the rapid decrease inimpedance of the primary circuit and the automatic decrease in effectivecoupling between the primary and 'secondary windings would cause thevoltage amplification to decrease with increase in the tuning frequency.

The resonant secondary circuit of transformer T10 ihcludes the primarywinding P2 of transformer T6 which is' magnetically coupled to thesecondary winding S2 thereof. The secondary winding S2 is, in turn,included in a resonant circuit 'Sz-Cs which contains the dissipativeimpedan'ce K14R12 proportioned to maintain an approximately constantresonance 'band width over the tuning range in the manner explainedabove. Thus, .the circuit-D when properly designed willhave asubstantially uniform overall frequency-gain characteristic, andlikewise a substantially constant'degree of selectivity over the tuningband. j

*Tlie double-tuned circuit D of Figure 2 is, of course,'an alternativearrangement to that of 'Figure '1, the transformer T5 in one instanceand which could be utilized to replace such transformers, as, fonexample, T4, T5 or T10. Examples of such circuitsare shown in Figures 3,4, and 5.

Ineach of these figures I represents the input terminals and O theoutput Jterminals. In Figure '3 there is provided only a magneticcoupling, between the primary winding P and the secondary winding S. Theprimary winding of relatively high inductance and is'tuned slightlybelow the tuning range.

- 1,961,154 by means of the natural capacity Km between turns or by aphysical condenser to a frequency The primary circuit has therefore afalling impedance characteristic over the tuning range with increase infrequency, and further. as the frequency increases there is an automaticdecrease in effective coupling between the primaryand secondary circuitsdue to the shunting effect of capacity K11. Both of these eifects offsetthe inherent tendency which would otherwise exist for the amplificationto increase with frequency.

In Figure 4 the primary circuit winding P is again wound to a relativelyhigh inductance and is tuned by means of the natural capacity K18between turns to a frequency slightly below the tuning band. In thisinstance, however, the primary circuit is coupled only capacitively tothe secondary circuit by'means of the coupling condenser K15 connectingthe high potential termi nals of the primary and secondary windings Pand S respectively. As the frequency of tuning increases, the primarywinding P, due toits falling impedance characteristic tends to producean ever decreasing efiect in the secondary winding S. Opposed to this,however, is the effect of the capacitive coupling between the primaryand secondary circuits, which increases with frequency, so that byproperly selecting the impedance characteristic of the primary winding Pand the magnitude of the coupling capacity K15,

' the frequen y-gain curve for the network may be sloped in a desiredmanner.

In the circuit of Figure 5 the primary winding P is of the same. generalorder of inductance as the secondary winding and is coupled magneticallythereto by means of mutual inductance M between windings,andcapacitively by means of condenser K16 extending between the highpotential terminals of the two windings. be observed that in thisinstance the primary winding is not resonant at a frequency below thetuning range, and accordingly its impedance will increase withfrequency, thus tending to give 'a positive slope to the gain-frequencycharacteristic. While the capacitive coupling Km may be arranged toeither aid or oppose the magnetic coupling or may not be employed atall, in general, it will be desirable to have the capacity couplingoppose the magnetic coupling in order to provide substantially uniformamplification over the tuning band.

The high gain circuit E of Figure 2 is arranged to have a substantiallyflat amplification.curve..

By the proper selection of the condenser Ks and resistor R0 theselectivity characteristic is made Y either uniform or slightly broaderat the low-'- frequency than at the high-frequency range.

Summing up the characteristics for the radio frequency amplifier, theoverall sensitivity will have the desired 2:1 ratio favoring the low frequencies, while the overall selectivity will be substantially uniformover the tuning range.

In Figure 2 the first stage of audio-frequency amplification shown inFig. 1 is omitted, the circuit F serving to connect the detector output.

directly through transformer Ts to the power tubes V1 and V0 connectedpush-pull.

With the type of receiverg'shown in Figures 1 or 2 the sensitivity ismade very high and to do this practically it is necessary to so positionthe parts, arrange the wiring and provide shielding sons to effectivelyeliminate extraneous couplings from-stage in stage and from the input tothe output.

receiver.

The various tuning coils are preferably individually shielded bygrounded metal cans. The amplifying tubes may be interposed between themetal cans to effectively shield the exposed portions from one anotheror they may be shielded by individual cans.

To eliminate common couplings in the chassisv pan, wires and-tuningcondenser, etc., it has been found extremely helpful to connect, forexaniple, the low potential terminal of each transformer secondarywinding directly to its own tuning Wherever possible, elements operatedat low condenser rotor by means of a separate wire and brush contactrather than to ground such terminals to the chassis pan and rely on theconnection from the pan to the condenser for a proper return path. Ithas also been found that'when employing by-pass condensers, such as forthe A, B and C voltages, and also the plate by-pass .for the detector,that these connections should be made directly to the cathode of thetube under consideration rather than to ground. In this way commoncouplings resulting. from currents flowing in the chassis pan areeliminated.

Couplings of the nature just discussed are especially important in adesign such as Figures 1 and 2, because the desired couplings betweenadjacent stages are small and extraneous or unknown couplings mighteasily vitiate the desired effects. v

I claim:

l; A high frequency electrical coupling system having input and outputterminals for interconnecting successive'portions of a multi-stagethermionic radio receiver, comprising, at least one resonant circuittunable throughout a. frequency range, and fixed -impedance meansincluded in said coupling system producing effects so variable withfrequency as to maintain the resonance band width of said couplingsystem substantially constant throughout said tunable frequency range.

2. A high frequency electrical coupling system having input and outputterminals for interconnecting successive portions of a multi-stagethermionic radio receiver, comprising, at least one resonant circuittunable throughout a frequency range, and fixed impedance means includedin saidcoupling system producing effects so variable with frequency asto render the selectivity 'of said coupling system when timed to afrequency adjacent the upper limit of said tunable range substantiallyequal to the selectivity of said coupling system tuned to a frequencyadjacent the lower limit ,of said tunable range.

3. A tunable high frequency electrical coupling circuit adapted tointerconnect successive. portions of a multi-stage thermionic radioreceiver,

comprising, a resonant secondary circuit, arid a primary circuitincluding a plurality of fixed impedance means so arranged and coupledto said secondary circuit as to produce eifectively in said secondarycircuit dualistic reactions proportioned to maintain the selectivity ofsaid cou- 5 pling system substantially constant throughout a tunablefrequency range.

4. A tunable high frequency electrical coupling y tem adapted tointerconnect successive portions of a multi-stage thermionic radioreceiver,

10, comprising, a resonant secondary circuit, and a primary circuitincluding a plurality of fixed impedance. means so arranged and coupledto said secondary circuit as to produce effectively in said secondarycircuit dualistic reactions which vary oppositely with frequency at suchrate as to maintain the selectivity of said coupling systemsubstantially constant throughout a tunable frequency range.

5. A tunable high frequency electrical coupling -system adapted tointerconnect successive portions of a multi-stage thermionic radioreceiver, comprising, a resonant'secondary circuit, and a primarycircuit including fixed impedance means so arranged and coupled to saidsecondary circuit as to produce effectively in said secondary circuitdualistic reactions which vary oppositely with frequency at such rate asto automatically adjust the effective coupling between primary andsecondary circuits from optimum or slightly more than optimum couplingat the low frequency end of the tuning range to less than optimumcoupling at the' upper frequency limit thereof for reducing variationsin selectivity otherwise present in said coupling system as the tuningis adjusted throughout the tunable frequency range. i

6. A tunable high frequency electrical coupling system adapted tointerconnect successive portions of a multi-stage thermionic radioreceiver, comprising, a resonant secondary circuit, and a primarycircuit including fixed impedance means so arranged and coupled to "saidsecondary circuit as to produce eiiectively in said secondary 'circuitdualistic reactions which vary oppositely with frequency at such rate asto automatically adjust the effective coupling between primaryandsecondary circuits from such a magnitude greater than optimumcoupling at the low fre quency end of the tunable range to such amagnitude less than optimum-coupling at the upper frequency end of thetunable range as to maintain the selectivity of said system constantthroughout the tunable frequency range. 75A high frequency electricalcoupling system including at least one resonant circuit tunablethroughout a frequency range, and fixed impedance means included in saidresonant circuit producing eil'ects so variable with frequency as toprovide approximately the same resonance band widths for said couplingsystem at the upper and lower frequency limits of said tunable range.

8. A high frequency electrical coupling system including at least oneresonant circuit. tunable throughout a frequency range, and fixedimpedance means included in said resonant circuit producing eifects sovariable with frequency as to minimize changes in selectivity of saidcoupling system as the tuning is adjusted throughout said frequencyrange.

9. A high frequency electrical coupling system including at least oneresonant circuit tunable throughout a frequency range, and energydissipating impedance means decreasing in dissipative effect withincreasein frequency included in said resonant circuit for reducingvariations in selectivity of said coupling system throughout saidtimable range. I

10. A high frequency electrical coupling system including at least oneresonant circuit tunable throughout a frequency range, and fixedimpedance means comprising resistance shunted by capacity seriallyinterposed in said resonant circuit for reducing variations inselectivity of said coupling system as the tuning is adjusted throughoutsaid frequency range.

:11. In an electric signaling arrangement a tunable coupling system forcoupling two portions thereof, adapted to respond to a uniform bandwidth of frequencies over a wide tuning frequency range, said couplingjsyst'em comprising'a pair of input terminals and a pair of outputterminals, an inductance element connected between said pairof inputterminals, a pair of adjustablyresonant circuits coupled together by adual coupling, a control member for simultaneously adjusting theresonance frequencies of said circuits, the first of said resonantcircuits being electromagnetically coupled to said inductance elementand the second of said resonant circuits being connected to said outputterminals, said dual coupling being. proportioned to provide a resultantcoupling which increases as the tuning frequency of said resonantcircuits is decreased, whereby the band width to which said system isresponsive remains substantially uniform over said tuning range.

12. A combination according toclaim 11 in which said dual coupling is acombined electrostatic and electromagnetic coupling.

13. A combination according to claim 11 in which said dual coupling issubstantially less than optimum when the resonant circuits are tuned tohigh frequencies and becomes greater as the tuning frequency isdecreased.

14. In an electric signaling arrangement, a pair of vacuum tubes eachhaving an input circuit and an output circuit, a tunable coupling systemadapted to respond to a uniform band width of frequencies over a .widetuning range, for coupling the output of one tube to the input of theother tube, said system comprising an inductance element connectedacross said latter output,.a pair of adjustably resonant circuitscoupled together by a dual coupling, a control member for simultaneouslyadjusting the resonance frequencies of said resonant circuits, the firstof circuits being electromagnetically coupled to said inductanceelement, the second of said resonant circuits being connected to saidinput circuit, said dual coupling being proportioned to increase as thetuning frequency of said resonant circuits is decreased, whereby theband width to which said system is responsive remains substantiallyuniform over said tuning range.

15. An electric coupling systemv comprising a pair of resonant circuitstunable to the same frequency over a wide range of frequency, a controlmember for simultaneously tuning'said circuits. means producing a pairof'couplings between said circuits which vary with frequency in anopposite manner relatively to each other, said couplings 140 beingproportioned to increase when the frequency of tuning is decreasedwhereby the widtl. of the frequency band transmitted by said system ismaintained substantiallyuniform over the I tuning range. I

18. An electric coupling system according to claim 15 in which said pairof couplings is an electromagnetic coupling and -'an electrostaticcoupling.

lLAn electric coupling system according touo said resonant 125 claim 15in which the means producing said pair of couplings are fixed.

18. An electric coupling system comprising a pair of resonant circuitstunable to-the same frequency over a wide frequency range, a controlmember for simultaneously tuning said circuits, means producing anelectromagnetic coupling and means producing an electrostatic cou-'pling, between said circuits, said couplings being proportioned toprovide a resultant coupling between said circuits which issubstantially less than optimum when said circuits are tunedto a,

high frequency and becomes greater as said circuits are tuned to lowerfrequencies.

19. In an electric signaling arrangement, a' tunable coupling systemadapted to respond to a uniform band width of frequencies over a widetuning frequency range, said coupling system comprising a pair of inputterminals and a pair of output terminals, an inductance'connectedbetween said pair of' input terminals shunted by such capacity that itis naturally resonant at a frequency 'lower but not greatly-lower thanthe lowest frequency of said tuning range, a pair of adjustably resonantcircuits coupled together by a dual coupling, a control ,member forsimul-v taneously adjusting the resonance frequencies of said circuits,the first of said resonant circuits being electromagnetically coupled tosaid inductance element and the second of said resonant circuits beingconnected to said output terminals, said dual coupling beingproportioned to provide a resultant coupling which increases as thetiming frequency of said resonant circuits is decreased, whereby theband width to which said system is responsive remains substantiallyuniform over said tuning range.

20. In an electric signaling arrangement, a

tunable coupling system adapted to respond to a uniform band width offrequencies over a wide V tuning frequency range, said coupling systemcomprising a pair of input terminals and a pair of output terminals, aninductance element connected betweensaid pair of input terminals, a pairof adjustably resonant circuits coupled together by a dual coupling, acontrol member for simultaneously adjusting the resonance frequencies ofsaid circuits, the first of said resonant circuits beingelectromagnetically coupled to said inductance element and the second,of said resonant circuits being connected to said output terminals, saiddual coupling being proportioned to provide a resultant coupling whichis over-optimum at the lower frequencies of said tuning range anddecreases as the tuning frequency is increased, whereby the band widthto which said system is responsive remains substantially uniform oversaid tuning range.

21. A high frequency electrical coupling system comprising a resonantcircuit, means for variably tuning said system over a-range of frequencyand means for causing the power factor of said system to increase whenthe frequency to which said system is tuned decreases.

22. A high frequency electrical coupling system comprising a resonantcircuit, means for variably tuning said system over a range in firequency and means for introducing resistance into said circuit whicheffectively increases in magnitude with decreasing frequency:

23. A high frequency electrical coupling system comprising a tunablecircuit, means for variably tuning said circuit over a range infrequency, means introducingresistance into said circuit and meansautomatically varying the effective magnitude of said resistanceinversely with respect to the tuning frequency, whereby a substantiallyconstant degree of selectivity is maintained over said frequency range.

24. A high frequencyelectrical coupling system' for translating a bandof signal frequencies from an input circuit to an output circuit, saidsystem comprising a variable tuning element for selecting the positionof said frequency band over 1;; a considerable range in frequency, aresistance, the magnitude of which is dependent upon the adjustment ofsaid tuning element and means for simultaneously varying said tuningelement and the magnitude of said resistance so that when said frequencyband is moved higher in the frequency-range the effective magnitude ofsaid resistance is decreased, whereby a substantially constant bandwidth may be maintained over said range.

25. A high frequency electrical coupling system tunable throughout arange in frequency, adapted to interconnect successive portions of amulti-stage amplifier, said'system comprising a DIBQLAiM EH1ipsiasijwilam 14'. u'wpmzd, Little Neck, N; Y. WA"? Swuamsc sums.

" Patentdated June 5, 1934, Disclaimer filed May 29, 1935, bythe'patentee,g

.jthe assignee,Haze1tine Corporation, assenting. Enters this disclaimerto .claims 2, 8, 9,10, 21, 22, 23, and l25 'of said patent which are inthe following words:'

26 A high fiequency electrical coupling system having input andoutput'ter- I minals for interconnecting 'successive portions ofa-multi+stage thermionic radio receiver, comprising, at least oneresonant circuit tunable-throughout a 'ire'quency range, and fixedimpedance means included in said coupling system producing efiectssovariable with frequency as to render the selectivity of sai couplingsystem when timed to-a uency adjacent the upper limit of said tunablerange substantially equal to thejseectivity of said coupling system.tuned to a frequency adjacent the lower-limit ofjsaid tunable range I VA j 8. Alfig frequency electrical coupling system includingat least oneresonant.

circuit tunable throughout a frequency range, and fixed impedance meansincluded in 7 said resonant circuit producing efiects so variable withfre uency as to mimr'nize.

c in selectivity of said coupling system as the timing is ad ustedthroughout saidfrequencyrange; r V q U 1 9. A high frequency electricalcoupling system including at least one resonant circuit Vhmalie'throughout a freqlency range, and energy dissipating impedance meansdecreasing in dissipative e ect with increase in frequency included'insaid resonant circuit or reducing variations in selectivity of saidcoupling system throughout said tunable range., K a

10. A high frequency electrical system including at least one resonantcircuit tunablethmughout a frequency range, and fixed impedancemeans:cqiiiprising shuhted by capacity serially interposed in saidresonant circuit for reducing variations in selectivity of said couplingsystem as the tuningis adjusted 'jth ioughoufl said frequency range.

"21. A High frequency-electricalcoupling'systemcom'prising a rdsonant?circuit,

. means for variablytuning said system over a range of frequency andmeans'i'oi'. causing the power of said system to increase when thefrequency'to which'saidsystem is v tuned decreasesiv A I 22. A highfrequency electrical coupling'system a resonant circuit, means forvariably tuning said system over a range in frequency and means forintroducing into said circuit whichefiectively increases in magnitudewith d ireqeuency.

23. A high frequency electrical coupling systemcomprising a tunablecircuit, means for.variabIy;tuning-said circuit over a ran '0 infrequency,- means'iintroducing resistance said circuit and meansautomaticafi of said redstauce inversely with respect to the tuning ireuencywh'ereby a substantially'constant degree of selectivity ismaintained over said frequency range.

y varying the effective. magnitude "-25. 'A liigh frequency electricalcoupling system tunable throughout a range in I uency,' adppted tointerconnect successive portionsiof a multi-stage'iamphfier, systemcbm.a primary circuit and a --tun'able' resonant secondary circuit.mductively .cqup ed'to said primary. circuit and, means associated withsecondary" circuit for introducing losses therein which'va as saidcircuit. is tuned throughout said range, to maintain substantiallyconstant t e'sele'ctivity of said system through-,-

Oilt saidltmblfltlency rangeff une18, 1935.

, D|sc A|M|=- R 1,961,154.William A. MacDonald, Little Neck, N. Y. WAVESIGNALING SYSTEM.

Patent dated June 5, 1934. Disclaimer filed August 20, 1935, by thepatentee,

the assignee, Hazeltine Corporation, assenting. Hereby enters thisdisclaimer to claims 1, 3, 4, 7 15,16, and 18 of said patent which arein the following words:

1. A high frequency electrical coupling system having input and outputterminals for interconnecting successive portions of a multi-stagethermionic radio receiver, comprising, at least one resonant circuittunable throughout a'frequency range, andfixed impedance means includedin said coupling system producing effects so variable with frequency asto maintain the resonance band width of said coupling systemsubstantially constant throughout said tunable frequency range.

3. A tunable high frequency electrical coupling circuit adapted tointerconnectsuccessive portions of a multi-stage thermionic radioreceiver, comprising, a resonant secondary circuit, and a primarycircuit "including a plurality of fixed impedance means so arranged andcoupled to said secondary circuit as to produce effectively in saidsecondary circuit dualistic reactions proportioned to maintain theselectivity of said coupling system substantially constant throughout atunable frequency range.

4. 1A tunable high frequency electrical coupling system adapted tolnterconnect successive portions of a multi-stage thermionic radioreceiver, comprising, a resonant secondary circuit, and a primarycircuit including a plurality of fixed impedance means so arranged andcoupled to said secondarycircuit as to produce effectively in saidsecondary circuit dualistic reactions which vary oppositely withfrequency at such rate as to maintain the selectivity of said couplingsystem substantially constant throughout a tunable frequency range.

7. A high frequency electrical coupling system including at least oneresonant circuit tunable throughout afrequency range, and fixedimpedance means included in said resonant circuit producing effects sovariable with frequency as to provide approximately the same resonanceband widths for said coupling system at the upper and lower frequencylimits of said tunable range.

15. An electric coupling system comprising a'pair of resonant circuitstunable to the same frequency over a wide range of frequency, a controlmember for simultaneously tuning said circuits, means producing a pairof couplings between said circuits wh1cl1 vary w 1thfrequency m anopposite manner relatively to'each other, said couplings beingproportioned to increase when the frequency of tuning is decreasedwhereby the widthof the'freguency band transmitted bys'aid system ismaintained substantially uniform over the tuning range.

16. An electric coupling system according to claim 15 in which said pairof couplings is an electromagnetic coupling and an electrostaticcoupling.

18. An electric coupling system comprising a pair of resonant circuitstunable to the same frequency over a wide frequency range, a controlmember for simultaneouslytunmg said circuits, means producing anelectromagnetic coupling and means producing anelectrostatic coupling,between saidcircuits, saidcouplings being proportioned toprovide aresultant coupling between said circuits which is substantially lessthan optimum when said circuits are tuned to a high frequency andbecomes greater as said circuits are tuned to lower frequencies.

[Ofiiaial Gazette September 10, 1935.]

